Hi
Yes, officially Dell T420 is supported up to version 6.5 U3. 6.7 install might work, however it might report issues/errors due to hardware incompatibility in near future..
You can try the following:
1. Connect to Dell Open Manage application/iDRAC interface and check the system event logs. If there are no relevant errors for action, please clear the system event logs.
VMware Knowledge Base
2. Ensure bios and other hardware firmware's are up to date.